Quotient circuit

ABSTRACT

A resistance drop feedback welding control circuit is disclosed which includes apparatus for detecting the effective instantaneous resistance across the welding position of a resistance welding machine from the electrode voltage and welding current. A method of detecting the effective instantaneous resistance from the electrode voltage and welding current is also disclosed. Means are provided for producing pulses in concurrence with peaks of the welding current waveform. Further means controlled by these pulses sample the electrode voltage and welding current at the welding current peaks. A quotient circuit produces a high frequency pulse train in which the pulse amplitude is proportional to the peak electrode voltage amplitude and the pulse duration is inversely proportional to the peak amplitude of the welding current. The quotient circuit includes means for averaging the amplitude of the high frequency pulse train to provide a DC output proportional in magnitude to the contact resistance at the welding position.

United States Patent [72] Inventor Peter W. Vanderhelst Primary Examiner-J. V. Truhe Livonia, Mich. Assistant ExaminerC. L. Albritton [21] Appl. No. 650,715 Att0meyWoodhams, Blanchard and Flynn [22] Filed July 3, 1967 [45] Patented Apr. 6, 1971 [73] Assignee Robotron Corporation ABSTRACT: A resistance drop feedback welding control C11- Detroit, Mich. cuit is disclosed which includes apparatus for detecting the effective instantaneous resistance across the welding position of a resistance welding machine from the electrode voltage and [54] QUOTIENT CIRCUIT welding current. A method of detecting the effective instan- 14 Claims 28 Drawing Figs taneous resistance from the electrode voltage and welding current is also disclosed. Means are provided for producing [52] US. Cl 219/110 pulses in concurrence with peaks of the welding current 1 f Cl 323k waveform. Further means controlled by these pulses sample Field Of the electrode voltage and welding current at the cur- 1 10 rent peaks. A quotient circuit produces a high frequency pulse train in which the pulse amplitude is proportional to the peak [56] References and electrode volta e am litude and the ulse duration is inversely 8 P P UNITED STATES PATENTS proportional to the peak amplitude of the welding current. 2,472,044 5/1949 Van Sciver 219/108 The quotient circuit includes means for averaging the am- 2,848,595 8/ 1958 Van Sciver 219/108 plitude of the high frequency pulse train to provide a DC out- 3,354,288 1 H1967 ONeal, .lr. 219/110X put proportional in magnitude to the contact resistance at the 3,422,243 l/ 1969 Meyer 219/110 welding position.

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. INVENTOR. I I I I I I I. PEAK E Mia/1 9% ATTOEMEYS QUO'IIENT CIRCUIT CROSS-REFERENCES TO RELATED APPLICATION 1. Application Ser. No. 650,789, 2. Application Ser. No. 650,964

FIELD OF THE INVENTION This invention relates to a resistance drop feedback welding control and more particularly relates to a method and apparatus for detecting the effective instantaneous resistance across the welding position from electrode voltage and welding current sensed thereat.

DESCRIPTION OF THE PRIOR ART It has been known for some time that, in the welding of plain uncoated mild steel, there is a discernible change in the electrical resistance of the workpiece as the weld is made. For the first cycle or two of weld time, the resistance is unstable as it is affected by the condition of the material surfaces. After three cycles or so, the surface conditions usually become stable and the resistance normally rises somewhat as the temperature of the workpiece rises. At the beginning of fusion the resistance levels off and begins to drop. It is this resistance drop that is indicative that a weld is being made. There is evidence that the amount of drop is related to nugget size.

Several controls are known which do nothing more than watch resistance under constant current conditions and terminate weld time when the desired resistance drop has occurred. Although .not too difficult to do, this has the disadvantage of requiring and causing widely varying weld times. This is usually not acceptable on a gun welding job or even on a high-speed press operation.

An improvement on this was disclosed by US. Pat. No. 2,348,595 to Van Sciver now assigned to the assignee of the present invention. In that system, the contact resistance at the weld position is constrained to follow a predetermined function during weld nugget formation. This function is experimentally deterrnined prior to welding and is programmed into the welding machine. The Van Sciver machine generates a function signal related to the above-mentioned predetermined function, compares the function signal to a signal related to the contact resistance detects, determines the error between them and then varies the heat input to the workpiece to correct such error.

However, the Van Sciver device has a number of operational disadvantages which it is the purpose of the present application and of copending application, Ser. Nos. 650,789 and 650,964 to cure.

The Van Sciver device is sharply limited in its use due to the types of components and circuitry used therein. It has been found that a device built according to the Van Sciver patent required the services of an engineer with an oscilloscope to set it up and it was necessary to repeat the set up every day due to tube drift and various other instabilities in the circuitry.

Accordingly, it is an object of this invention to provide a feedback resistance drop welding control including an improved method and apparatus for detecting the instantaneous contact resistance at the welding position.

A further object is to provide a method and apparatus, as aforesaid, particularly adapted and intended for use with circuitry for comparing said instantaneous resistance to a preselected resistance function capable of causing the instantaneous contact resistance to reach a preset value at a preset time and for use with further circuitry capable of causing the heat applied to the welding position to vary in a manner to reduce its deviation from the preselected function.

A further object is to provide a method and apparatus, as

, aforesaid, in which the value of the instantaneous contact resistance is obtained by taking the quotient of the instantaneous peak electrode voltage at the welding position and the instantaneous peak welding current appearing at the welding position. A further object is to provide a method and apparatus, as aforesaid, in which the time of occurrence of the peak in the welding current waveform is noted and in which the values of the welding electrode voltage and welding current used to determine the instantaneous weld contact resistance are sensed at the peak of the current waveform.

A further object is to provide a method and apparatus, as aforesaid, in which error in the value of the electrode voltage due to induction effects is eliminated by detecting electrode voltage at the peak of the welding current waveform.

A further object is to provide a method and apparatus, as aforesaid, in which the instantaneous value of the electrode voltage and of the welding current at the peak of the welding current waveform are stored between current peaks and in which the instantaneous contact resistance is determined from these stored values.

A further object of this invention is to provide a method and apparatus, as aforesaid, in which signals which are a function of the electrode voltage and the welding current amplitudes are caused to independently control two independent parameters of a pulse waveform which waveform is a function of the contact resistance.

A further object is to provide a method and apparatus, as aforesaid, in which a train of pulses is produced in a manner that the amplitude of each pulse is a function of the instantaneous value of the electrode voltage sensed, the duration of each pulse is a function of the instantaneous value of the peak current sensed and the average amplitude value of the pulse waveform is a function of the contact resistance.

It is a further object of this invention to provide an apparatus, as aforesaid, which is highly stable in operation, which is insensitive to changes in its operating environment, which will operate for .long periods without adjustment, which can be readily and inexpensively manufactured from standard components, which can be compactly packaged, which is adaptable to use with a wide variety of welding machines and which is capable of a long and trouble-free service life with little or no maintenance.

Other objects and purposes of this invention will be apparent to persons acquainted with the apparatus of this general type upon reading the following description and inspecting the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram of a feedback heat correction circuit embodying the present invention.

FIG. 2 is a schematic diagram illustrating the peak pulse circuit of FIG. 1.

FIGS. 3A-3F are waveform diagrams related to signals appearing at various points in the peak pulse circuit of FIG. 2.

FIG. 4 is a schematic diagram of the peak I circuit of FIG. 1.

FIGS. 5A5D are waveform diagrams illustrating signals appearing at various points in the peak I circuit of FIG. 4.

FIG. 6 is a schematic diagram of the peak E circuit of FIG. 1.

FIGS. 7A-7F are waveform diagrams illustrating signals appearing at various points in the peak E circuit of FIG. 6.

FIG. 8 is a schematic diagram of the quotient circuit of FIG. 1.

FIGS. 9A-9G are waveform diagrams illustrating signals appearing in the quotient circuit of FIG. 8.

Certain terminology will be used in the following description for convenience in reference only and will not be limiting. The words upwardly", downwardly, rightwardly and Ieftwardly will refer to directions in the drawings referred to. Such terminology will include the words above specifically mentioned, derivatives thereof and words of similar import.

SUMMARY OF THE INVENTION In general, the objects and purposes of this invention are met by providing a resistance drop feedback welding heat control circuit which compares the instantaneous value of the contact resistance at the welding position with a preselected resistance function and controls welding heat to minimize the difference between the instantaneous contact resistance and the instantaneous value of said resistance function. For providing the instantaneous value of the contact resistance, the welding current waveform and the electrode voltage waveform at the welding position are sensed. Means, fed by the welding current waveform, produce a pulse train, in which pulses occur at the peaks of the welding current waveform. In addition, circuitry is provided for reading the instantaneous amplitude of the electrode voltage waveform and the welding current waveform coincidentally with the occurrence of pulses in the pulse train. The values read are stored and supplied to quotient circuitry. The quotient circuitry includes means for generating a second pulse train and means for controlling the amplitude and duration of each pulse of the second train as a function of respective ones of the stored electrode voltage and welding current amplitude values. The instantaneous value of the contact resistance is formed by averaging the amplitude of pulses of the'second train.

DETAILED DESCRIPTION Turning now to the drawing, FIG. 1 discloses in block diagram form the feedback resistance drop welding control embodying the invention and which causes the contact resistance at the weld position during welding to follow a preselected desired resistance function. The resistance drop feedback control 10 includes a portion 11 for providing a signal proportional to the instantaneous contact resistance hereinafter referred to as the contact resistance generator 11. The contact resistance generator 11 includes an electrode voltage sensing circuit 12 and a current sensing circuit 13 which are connected to the welding circuit W to sense the electrode voltage and welding current, respectively, and to provide continuous voltage signals E and I of waveform corresponding to the electrode voltage and welding current, respectively. Theoutputl, of the current sensing circuit 13 is fed to a peak I pulse circuit 14 which generates a train of narrow pulses synchronized with and appearing at the half-wave amplitude peaks of the welding current. The outputs E and I, of the electrode voltage sensing circuit 12 and the current sensing circuit 13 are fed respectively to a peak E circuit 16 and a peak I circuit 17. The pulse train output of the peak I pulse 14 is applied through lines 21 and 22 to the peak E circuit l6 and peak I circuit 17 to cause same to detect the instantaneous values of the electrode voltage signal E and welding current signal I from the circuits l2 and 13, respectively, in synchronism with the amplitude peaks of the welding current waveform. The detected values E and I of the electrode voltage and welding current signals E and I respectively, are applied through lines 23 and 24, respectively, to the input of a quotient circuit 26. The quotient circuit 26 determines the quotient peak E/peak I=r. The signal r is impressed, through a resistance 27, on the input of an operational amplifier 31. The output signal R of the operational amplifier 31 is thus an analog representation of the instantaneous contact resistance at the welding position taken at successive peaks of the weld current.

The feedback control circuit 10 further includes an end weld control circuit 36 preferably of the type disclosed in copending application Ser. No. 650,789. The output R of the amplifier 31 is applied to the end weld control 36 through a line 37. In addition, the end weld control 36 receives a signal from the welding timer WT, which timer may be of any convenient type, for example, one of the type shown in copending application, Ser. No. 369,727 assigned to the assignee of the present invention. Thus, the timer WT impresses on the end weld control 36, through a line 41, a signal which indicates the beginning of weld current flow through the weld position W during each half cycle of weld time. Customarily, the contact resistance will build up from a relatively low value at the beginning of the welding interval to a maximum value as the weld nugget forms and then will drop off at a substantially, though not necessarily precisely, linear rate until the weld is completed. The end weld control 36 prevents the control 10 from operating during the first few cycles (termed the transition period) of the weld-time during which time the contact resistance is unstable due to material surface conditions. The end weld control 36 has an output signal which is applied to an output line 42 at the end of the transition time. A further function of the end weld control is to detect the highest value R, of the contact resistance signal R reached after transition time with succeeding instantaneous values of R. The end weld control is arranged to impress an end weld signal on a further output line 43 which leads back to the timer WT when a predetermined ratio of the instantaneous value R and peak value R, is reached. In the event that such a ratio is not achieved within a preselected time the timer WT further actuates the end weld control 36 to indicate the end of the weld interval and the end weld control is then adapted to actuate an alarm of any convenient type to signal that a questionable weld has taken place.

The control 10 further includes a heat correction control circuit 46 preferably as shown in my copending application Ser. No. 650,879. The heat correction control 46 in turn actuates and controls a conventional heat control circuit 47 for varying the heat input to the welding zone in a-manner to minimize the difference between the value of the instantane' ous contact resistance R and a desired resistance function generated therewithin. To this end, the transition signal line 42 from the end weld control 36 connects to the heat control 46 to initiate its operation. Further, the output of the amplifier 31 is taken through a line 48 to impress the contact resistance signal R on the heat correction control.

The peak E signal fed to the quotient circuit 26 may be fed by a line 49 to the heat correction control circuit 46 when it is desired to regulate welding heat with reference to the electrode voltage rather than the contact resistance.

Turning now more particularly to the contact resistance generator portion of the feedback control 10, FIG. 2 discloses the preferred embodiments of the peak 1 pulse circuit 14 of FIG. I, and of the current sensing circuit 13. The current sensing circuit 13 includes a current transformer 52 having a primary winding 53 connected in series with the primary or secondary of the welding transformer 54 and the welding electrodes E of the welding position W for energization by welding current flowing therethrough. The secondary winding 56 of the current transformer 52 connects across the primary 57 of a further transformer 58. A rheostat 59 shunts the secondary 56 of the current transformer to adjust the magnitude of the current signal I impressed on the primary winding 57 of the input transformer 58 of the peak I pulse circuit 14 so that the input signal will fall within the range of the circuit 14. Lines 6! connect the ends of the secondary 56 of the current transformer 52 to the input of the peak I circuit of FIG. 4 as hereinafter discussed.

The secondary winding 62 of transformer 58 is center tapped. The center tap thereof connects to a common poten tial line 63 while each of the ends of the secondary 62 connects through a diode 66 to a junction point 67, the cathodes of diodes 66 both facing the junction point. A load resistor 68 connects between the junction point 67 and the common line 63. Thus, the current signal I applied to the primary winding 57 of the transformer 58 is transformed to the proper voltage level and full wave rectified by diodes 66. The rectified signal is applied across the load resistor 68.

The peak I pulse circuit 14 includes a trigger circuit 70 comprising transistors 10 and 20. Junction point 67 is coupled by a capacitor 69 to the base of transistor 10. Base bias is supplied to transistor 1Q by a resistor 71 from the common line 63. A diode 72 clamps the base of transistor 10 at or above the potential on common line 63.

A swamping circuit 73, hereinafter described, provides a swamping pulse to the base of transistor 10 as the ignitrons driving the weld transformer 54 turn on during each half cycle of the weld interval to swamp out any noise which might reach transistor IQ The emitter of transistor connects directly to the common line 63 and its collector connects through a dropping resistance 74 to a positive potential line 76 held by any convenient power supply, not shown, at a specified positive volt age, here for example 30 volts. A further transistor is connected at its emitter to the common line 63, at its collector through a dropping resistance 77 to the positive potential line 76 and at its base to the collector of transistor 10.

The peak pulse circuit 14 further includes a one-shot multivibrator circuit 80 comprising transistors 30 and 40. A series resistance 78 and capacitance 79 extends from the collector of transistor 20 and provides a signal to the base of transistor 3Q. Base bias is provided to the transistor 30 from the common line 63 through a resistor 81. The emitter of transistor 30 connects directly to the common line 63 and through a dropping resistor 82 to the positive potential line 76. Output is taken from the collector of transistor 30 through a series resistance 83 and capacitor 84 to the base of transistor 40. Base bias is applied to transistor 40 through a resistance 86 from the common line 63. The emitter of transistor 40 connects directly to the common line 63 and the collector thereof connects through the primary winding 87 of an output transformer 88 to the positive potential line 76. The primary winding 87 is shunted by a diode 89, the anode of which connects to the collector of transistor 40. Resistances 91 and 92 connect from the positive potential line 76 to the base of transistor 3Q and cooperate with resistor 81 in biasing same. In addition, feedback is applied from the collector of the transistor 4Q to the base of transistor 3Q through a diode 93 in series with the resistance 92, the anode of diode 93 facing the resistance 92. The output 1,, of the circuit 14 is a pulse train which is applied to output transformer 88, the pulses starting at successive peak points of the weld current half cycles.

Considering the swamping circuit 73 in more detail, same includes transistors 50 and 60. The transistor 50 is an emitter follower, its collector being connected to the positive potential line 76 and its emitter being connected through a dropping resistor 96 to the common line 63. Base bias is supplied through a resistor 97 from the positive potential line 76.

The weld timer WT controls the flow of weld current through the electrodes. The weld timer WT supplies a signal when the ignitrons, not shown, energizing the weld transformer 54 begin conduction in each half cycle of weld current. The signal in the particular embodiment shown is a negative pulse (FIG. 3D) which is applied through a diode 98 to the base of transistor 50 to reduce its conduction. Output is taken from the emitter of transistor 50 is fed through a resistor 99 to the base of amplifier transistor 6Q. Base bias is applied to transistor 6Q through a resistance 101 connected to the common line 63. The emitter of transistor 6Q connects directly to common line 63 and its collector is connected through a resistor 102 to the positive potential line 76. A positive pulse appears on the collector of transistor 60 when conduction through the ignitrons is desired to begin and the system is relatively noisy, and is applied through a series diode 103 and resistor 104 to the base of transistor 10 as aforementioned.

The peak I circuit 17 (FIG. 4) includes a transformer 106.

i The current signal I is applied to the primary winding 107 of a I transformer 106 by the lines 61 (FIG. 2) from the current sensing circuit 13. The transformer ratio is selected to provide the proper signal magnitude for the rest of the peak I circuit 1 17. The secondary winding 108 of the transformer 106 is center tapped, the center tap thereof being connected to the positive potential line 76. The ends of the secondary winding 108 connect through similar orientated diodes 109 to a junction point 111, the anodes of the diodes 109 facing the junction point 111. The full wave rectified current signal is of negative polarity with respect to the positive potential line 76 and appears across the dropping resistance 112 connecting the junction point 111 to the positive potential line 76.

A pair of transistors 270 and 28G form a series gate circuit 113. The series gate circuit 113 includes a voltage divider comprising series resistors 114 and 116 connecting the bases of transistors 270 and 280. The emitters of transistors 270 and 280 are directly connected by a conductor 117. The secondary winding 118 of the peak 1 pulse transformer 88 (FIG. 2) connects from the center of the voltage dividerll4, 116 to the emitter line 117 for applying the peak 1 pulse signal 1 to the bases of transistors 270 and 280.

The cathodes of shunting diodes 121 and 122 are connected to the bases of transistors 270 and 280, respectively, and the anodes thereof are connected to the emitter line 117 for shorting reverse base-emitter voltages to protect said transistors. The input end of the gate 113 comprises the collector of transistor 270 which is connected to the junction point 111 above mentioned. The output end of the gate 113 comprises the collector of transistor 280 which is connected to the input of a buffer circuit generally indicated 123, here a triple emitter follower. Each peak I pulse on the secondary 118 of the peak I transformer 88 causes the transistors 270 and 280 to conduct and thereby opens the gate 113 for the duration of the peak 1 pulse. The signal on junction point 111 is applied by the open gate 113 to the base of the first transistor 8Q of the buffer 123.

The buffer 123 also includes further transistors 9Q and 100. The collectors of transistors 80, 9Q and 100 directly connect to the positive potential line 76. The emitters of transistors and are connected to the bases of succeed ing transistors 90 and 10A, respectively, and the emitter of transitor 10Q is connected by a dropping resistor 124 to the common line 63. The signal applied to the input of the buffer 123 when the gate 113 opens is applied to a memory capacitor 126 connected from the base of transistor 80 to the common line 63. The output of the peak I circuit 17 is taken between the emitter of the output transistor and the positive potential line 76. This output, as shown in FIG. 5d, is a step function, the magnitude of which is proportional to the magnitude of the peak amplitude of the current signal 1 and therefore to the weld current. An increase in weld current amplitude increases the magnitude of the step function peak 1 in a negative direction.

The electrode voltage sensing circuit 12 (FIG. 6) energizing the peak E circuit 16 includes a voltage transformer 131 having a primary winding 132 which is connected through conductors 133 to the welding electrodes WE adjacent the weld position W. The welding electrode voltage is transformed to a magnitude suitable for energizing the peak E circuit 16 and appears across the center tapped secondary winding 134 of the transformer 131. The center tap 136 is connected to the input of a gate circuit 1130 including transistors 290 and 30C). The gate 113a preferably is similar to the gate circuit 113 described above with respect to the peak I circuit 17 of FIG. 4

and thus requires no further description. The ends of the secondary winding 134 connect to the cathode ends of diodes 137 and the anode end of the diodes 137 connect to a junction point 138. A resistor 139 is connected across the center tap 136 and junction point 138 and carries the full wave rectified output (FIG. 7D) of the transformer 131.

The junction point 138 is connected to the common potential line 63. As in the peak I circuit of FIG. 4, a capacitor 1260 is connected between the common line 63 and the output of gate circuit 1130. The output of the gate circuit 113a appears across the capacitor 1260, is fed through a buffer circuit 1230 connected between the common line and the positive potential line 76 and comprising cascaded, follower transistors 19Q, and 170. The buffer circuit 123a is preferably similar to the buffer circuit 123 of FIG. 4, above described and hence requires no further description. However, the peak E circuit differs from the peak I circuit 16 of FIG. 4 in that the dropping resistor 139, to which the rectified signal from the input transformer is supplied, connects the gate input terminal to the common line 63 rather than to the positive potential line 76. The peak E circuit 16 further differs in that its output is taken across the emitter resistor 1240 of the buffer 123a rather than between the emitter resistor and the positive potential line a s in the peak I circuit. As in the peak 1 circuit of FIG. 4, the output of the peak E circuit is a step function (FIG. 7F), but one which swings positive as the magnitude of the electrode voltage E increases, the magnitudes of the step function and electrode voltage being proportional.

The quotient circuit (FIG. 8) receives inputs by lines 24 and 23 from the peak I and peak E circuits l7 and 16, respectively. The peak I line 24 connects to the base of the transistor 70. Transistor 70 is opposite in polarity to the other transistors in the circuit 26 and is therefore in the present embodiment, a PNP transistor. The emitter of transistor 70 connects through a dropping resistor 141 to the positive potential line 76 and the collector thereof connects through a capacitor 142 to a negative potential line 143, here maintained at negative 30 volts with respect to the common line 63.

The charge applied to the capacitor 142 by conduction through the transistor 70 is directly applied to and controls the potential of the emitter of a unijunction transistor 11C; The upper base of unijunction 11Q connects through a dropping resistance 146 to the common line 63 and its lower base connects through a dropping resistance 147 to the negative potential line 143.

A further unijunction transistor 140 is connected as a freerunning oscillator, its bases being connected through resistors 148 and 149 to the common and negative potential line 63 and 143, respectively. The emitter of the unijunction transistor 14Q connects between a resistor 151 and capacitor 152 in turn serially connected between the common line 63 and the negative potential line 143. The capacitor 152 charges through its series resistor 151 to a level sufficient to tire the unijunction 140, which discharges the capacitor for another cycle. The RC constant of the series resistor 151 and capacitor 152 is chosen to give an oscillator frequency well above the weld current frequency, e.g., 10. kHz. This high frequency output is taken from the lower base of transistor 140 through a series resistance 153 and diode 154, the cathode of diode 154 being connected to the base 155 of a transistor 130.

The transistor 130 and a further transistor [20 form a flipflop circuit 156. The emitters of transistors I20 and 130 connect directly to the negative potential line 143 and the collectors thereof connect through dropping resistors 157 and 158, respectively, to the common line 63. Additionally, the collector of transistor 12Q connects through a parallel capacitance 161 and resistance 162 to the base of transistor 130 and the collector of transistor 130 similarly connects through parallel capacitance 163 and resistance 164 to the base of transistor 120. In addition to the oscillator signal applied to the base 155, a further input is made to the flip-flop circuit 156 through a series resistance 166 and diode 167 for applying positive potentials to the base of transistor 120 from the lower base of unijunction 110 above described. Base bias is provided to the transistors 120 and 130 from the negative potential line 143 through resistances 168 and 169, respectively. The collector of flip-flop transistor 120 also connects through an inductor 171 in series with a diode 172 to the upper end of capacitor 142 to maintain same discharged when transistor 120 is conductive.

The output line 23 from the peak E circuit 16 of FIG. 6 connects through an isolating diode 176 to the collector of a transistor 160, whereby the collector potential of the transistor 160, when same is nonconductive, is directly proportional to the magnitude of the peak E step signal. The collector of transistor 160 is biased through a dropping resistor 177 from the positive potential line 76, the emitter of said transistor being directly connected to the common potential line 63. The collector of transitor 130 connects through a conductor 178 in series with parallel resistor capacitor network 181 and 182 to the base of transistor 160 to vary the time of nonconduction of said transistor inversely proportional to the magnitude of the peak I signal. Base bias is provided the transistor 160 from the positive potential line 76 through a resistor 183.

The collector potential of transistor 160 is applied to the base of an emitter follower transistor Q through a line 184.

The collector of transistor connects to its positive potential line 76 and its emitterv connects through a dropping resistance 186 to the common line 63. Output is taken from the emitter of transistor 150 across the resistor 186. This output is a square wave train indicated in FIG. 9 in which the amplitude E of each pulse is proportional to the welding electrode voltage magnitude while the duration H] is inversely proportional to the weld current magnitude. This square wave train is applied to the input of an integrating network 191, here comprising a first inductance 192 and capacitance 193 connected in series across the resistor 186 and second series inductance 194 and capacitance 196 connected across the first capacitor 193. The network 191 provides a DC voltage output r proportional to the instantaneous contact resistance at the weld position. Output is taken from the network 191 across the second capacitance 196 and applied to the input of a high gain and polarity inverting operational amplifier 197 of any desired type. A resistance 198 is series with the capacitor 196 across the input of the amplifier 197 controls the amplitude of the input signal thereto. The operational amplifier 197 has the usual negative feedback resistance loop 199 from the output to the input therefor for regulating the gain thereof. The output R of the operational amplifier 197 appears on line 200 and is proportional to the instantaneous contact resistance across the welding position.

OPERATION Although the operation of the present invention has been indicated to some extent hereinabove, the operation will hereinafter be summarized to assure a complete understanding of the invention.

1 The Peak Pulse Circuit The peak I pulse circuit 14 generates signal pulses I (FIG. 3F) which occur in each half cycle of weld current, at the peak of the weld current wave where the current is changing very slowly. It is at this point in the wave where the inductive signal coupled to the electrode voltage sensing wires is nearly zero.

A voltage signal I, (FIG. 3A) corresponding in waveform to the weld current is derived from current transformer 52 and applied through lines 61 both to the peak I circuit 17 and to transformer 58. Rheostat 59 connected across the secondary 57 adjusts the magnitude of the current signal so that it will fall within the working range of the control 10. The current signal I is transformed to the proper voltage level, full wave rectified, applied across load resistor 68 and capacitively coupled to the base of transistor IQ. As long as the current signal I is increasing, the coupling capacitor 69 will be charging and base current will flow in transistor 10 causing this transistor to conduct. When the current signal I reaches its peak level, charging of the capacitor 69 ceases and transistor 10 ceases to conduct.

Transistors IQ and 20 are coupled together in a trigger circuit 70 so that the collector voltage (FIG 3C) of transistor 20 will be a reasonably sharp square waveform which goes positive at the beginning of weld current flow in each half cycle and goes negative at the peak of weld current.

Now at the beginning of weld current flow in each half cycle of the weld interval the current signal I is rising rather slowly. Also, at this time, noise and transient signals are being produced in the system from a variety of causes. Some of this noise could conceivably be coupled into the trigger circuit 70 composed of transistors 10 and 2Q to cause false triggering which would be most undesirable. For this reason, auxiliary signal circuit 73 is used to make sure that transistor 10 conducts when weld current is intended to commence flowing in each half cycle of the weld interval.

A signal (FIG. 3D) that the ignitron tubes (not shown) are about to begin conduction in each half cycle of weld time is derived from the weld timer WT. This firing signal is here a positive voltage which goes negative as the weld current control system in the timer WT is triggered to turn on the weld current. The negative firing pulses are of sufficient duration (here several hundred microseconds) to ensure that the usual noise" generated in the welding system will have abated during the pulse duration. The firing pulse is applied to emitter follower transistor 50 (for isolation purposes), and inverted in sign by simple amplifier transistor 60 to create a positive going swamping" pulse (FIG. 3E) which is applied to the base of transistor 10 during the critical ignitron turn on time to swamp any noise which might reach the latter transistor. False triggering of transistor 10 due to system noise generated by ignitron contactor tum-on is thereby prevented.

The collector voltage of transistor (FIG. 3C) is capacitively coupled to the one-shot" multivibrator 80 comprising transistors 30 and 40. The drop of this collector voltage causes transistor 3Q to cut off and transistor 40 to conduct for a period of time determined by the circuit constants to apply a pulse I (FIG. 3F) of current to the primary of transformer 88. This pulse I is known as the peak pulse" since it occurs at the magnitude peak of the weld current half cycle.

2. The Peak I Circuit The peak I circuit 17 (FIG. 4) generates a steplike signal proportional in magnitude to the peak magnitude of weld current. The gate 113 is actuated by the peak pulse I through transformer 88 to take a-sample of the weld current signal I at its peak point and store this value on capacitor 126 until the peak point of the next half cycle at which time another sample will be taken.

More particularly, a portion of the weld current signal I,. from the currenttransformer 52 (FIG. 2) is transformed to the proper magnitude, full wave rectified, and applied across load resistor 68. Transistors 270 and 28Q of gate 113 are nonnally cutoff due to the absence of base drive but are turned on by the peak pulse signal I,,, (FIG. 5C) from the secondary winding 118 of transformer 88. The voltage stored in the storage capacitor 126 is, therefore, equal to the peak value of the current signal in each half cycle. The stored voltage is buffered out by the triple emitter follower current amplifier 123 made up of transistors 8Q, 90 and [0Q to provide current capability and appears on emitter line 24 as the peak I signal (FIG. 5D).

Note that the return path for the current signal rectifier circuit is to the positive line 76. The peak I signal therefore is measured as a negative-going signal with respect to the positive line 76.

3. The Peak E Circuit The peak E circuit 16 (FIG. 6) developes a signal voltage (peak E) proportional to the voltage across the welding electrodes (FIG. 7A) at the current peak point and operates similarly to the peak I circuit 17 above described.

More particularly, conductors 133 connect the welding electrodes WE to the primary winding of transformer 13. The voltage signal E (FIG. 7D) is transformed to the proper value, full wave rectified, and applied across load resistor I39. Normally, nonconductive transistors 290 and 300 of series gate 113a are turned on by the peak pulse I from another secondary winding 1180 of transformer 88 to charge the peak E storage capacitor 126a. The voltage stored on this capacitor is buffered out by transistors 170, I80, and [90 and appears across resistor 124a as a step signal peak E (FIG. 7F), proportional to the peak electrode voltage. This signal is a positivegoing signal with respect to common line 63.

It will be noted that the voltage across the welding electrodes (FIG. 7A) is comprised of a series of at least approximately sinusoidal half waves of alternating polarity which is in phase with the welding current. However, due to inductive effects, the conductors 133 pick up an additional spurious signal (FIG. 7B) proportional to the rate of change of welding current flow di/dr. The spurious signal has maximum points at the zero magnitude points of the welding current waveform where di/dr is at a maximum and goes through zero at the weld current peaks where di/dt is zero. Thus, the signal E applied to the transformer 131 is a composite of the welding voltage and the spurious, induced voltage'and as seen in FIG. 7C, the maxima and minima of the composite voltage signal E are not in phase with the peaks of the actual electrode voltage and the welding current. To obtain a peak E value proportional to the peak value of the actual welding voltage of FIG. 7A, the maxima of the rectified composite signal E (FIG. 7D) must be ignored. Instead, the rectified signal E, must be sampled at the weld current peaks where the inductive effect is not present, i.e., where di/dt= 0, and where also the weld voltage is at maximum magnitude. For this reason, the gate 113a is opened at the weld current peaks by the peak I pulses I}, whereby the potential impressed on capacitor 126a is proportional to the peak voltage at the welding electrodes WE and is unaffected by the spurious inductive signal.

4. The Quotient Circuit The quotient circuit 26 (FIG. 8) divides peak E by peak I and yields a signal proportional to weld resistance R.

Unijunction transistor I40, connected as a free-running oscillator, generates positive pulses, at a frequency substantially higher than the welding current frequency, to turn on transistor I30. Transistors 130 and 120 are connected in a flip-flop" circuit. Conduction of transistor H30 causes transistor I2Q to turn off so as to unclamp the capacitor 142 connected to the emitter of unijunction transistor HQ and allow it to charge. Transistor 70, by virtue of emitter circuit feedback, acts as a current regulator to charge the above-mentioned capacitor at a rate detennined by the magnitude of peak I. The charge time to any specific voltage will, therefore, be inversely proportional to the magnitude of peak I. Transistor 70 charges the aforementioned capacitor to the peak point voltage of unijunction transistor at which point unijunction llQ conducts to generate a positive-going pulse at its base 1 electrode to cause transistor to turn on. The high characteristic stability with respect to time of the unijunction transistor assists close correlation of time during which capacitor 142 is charging and the magnitude of peak I. Conduction of transistor 12Q clamps the charging circuit of transistor 70 so that no further charging will take place until oscillator 140 generates another pulse at which time the process will repeat itself.

The time period of conduction of transistor is, therefore, inversely proportional to the magnitude of peak I. The frequency of oscillation of transistor is high with respect to frequency of variation of peak I signal and is here approximately 10 kc whereas the input signal normally varies at a rate of 120 c.p.s. maximum. The collector voltage of transistor 130 is then in this embodiment a pulse train of frequency 10 kc wherein the width of the negative-going portion of each pulse is modulated by the instantaneous value of the peak I signal so that this width is inversely proportional to the magnitude of the welding current.

The frequency of the oscillator 14Q is preferably substantially higher than the weld current frequency to allow the two frequencies to be readily separated with the relatively simple filter 191 so as to eliminate the oscillator frequency from the output of the quotient circuit. Further, a high oscillator frequency is desirable to achieve quotient circuit response soon after the weld current peak occurs and the weld current and electrode voltage are remeasured, in the present embodiment within 8 milliseconds or so of the weld current peak.

The collector voltage of transistor 130 is applied as a signal to the base of transistor 160 to cut same off when transistor 130 conducts. During these nonconductive periods, the collector voltage of transistor 16Q will rise to the magnitude of peak E due to clamping by diode 176. This signal is buffered out by emitter follower transistor to yield a pulse train signal (FIG. 9) with amplitude proportional to weld peak voltage and width inversely proportional to weld current. The average DC value of this signal is, therefore, proportional to weld resistance and is known as r. This is a low-level signal so it is amplified by a resistance feedback stabilized amplifier to yield weld resistance signal R. This resistance signal R is a positive going signal measured with respect to DC common line 63.

Proportioning the pulse duration inversely to the weld cairrent amplitude rather than to the electrode voltage desirably renders the output signal R= when weld current and electrode voltage are both zero and the expression R=E/l would otherwise be indeterminate.

The present quotient circuit avoids inaccuracies in prior circuits by using variables (time and signal amplitude) to which the quotient taking device 16Q responds independently and linearly. This arrangement eliminates possible inaccuracies in representing the inputs with similar variables (e.g., signal amplitudes) applied to different electrodes of a single device wherein a change in one signal may undesirably modify the response of the of the device to the other signal, as by moving the operating point of the device into a nonlinear region.

The R generator circuitry 11 and the method of generating the R signal output hereinabove described are particularly intended and designed for use with the end weld control 36 and heat correction control 46 and as further discussed in the above-mentioned copending applications, Ser. Nos. 650,789 and 650,964 respectively.

The present R generator circuitry 11 is of high stability and provides a DC output signal R which precisely tracks the contact resistance at the weld position during welding. The signal R as above discussed is not influenced by noise of other instabilities in the central portion of weld current flow, by inductive effects in the measurement of electrode voltage or by spurious interactions of the variables in the quotient taking process. Still further, the ratio R=E/l is computed a number of times in each half cycle at a rate determined by the oscillator 140, rather than just once, which desirably allows the quotient R to be computed quickly after each current peak (here within the first 8 milliseconds or so). Thus, the R generator 11 is particularly adapted for determining instantaneous weld contact resistance for the succeeding end weld control and heat correction control due to its rapid reaction to changes in resistance and avoidance of sources of error likely to be encountered in measurements of weld contact resistance and which have undesirably influenced prior measuring circuits.

In summary, therefore, the R generator 11 generates the voltage signal E and current signal I in the circuits 12 and 13. The peak I pulse circuit 14 produces timing pulses I,,,, when energized by the current signal l and applies such timing pulses to the peak E circuit 16 and peak 1 circuit 17 also energized, respectively, by the electrode voltage signal E and current signal l Thus, the peak E and peak I circuits l6 and 17 measure and store the instantaneous amplitude of the signals E and I respectively, when energized by the timing pulse l,,,, at the peak of the welding current waveform. The values so measured and stored, termed the peak E and peak I signals, are fed to the quotient circuit 26 which produces a DC output corresponding to the quotient peak E which is there after amplified. The preferred quotient circuit 26 generates a high frequency pulse train, the pulse duration of which is constrained to proportionality with the inverse of the weld current magnitude and the amplitude of which is constrained to proportionality with the welding electrode voltage whereby the integrated value of each pulse is the desired resistance signal r proportional to the contact resistance.

Although a particular preferred embodiment of the invention has been described above for purposes of illustration, modifications and variations thereof lying within the scope of the present claims are fully contemplated.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

Iclaim: l. A method for finding the contact resistance of a resistance welder comprising the steps:

sensing the amplitude of the electrode voltage and welding current; producing a train of pulses, the amplitude of each pulse corresponding to one of the amplitude of the electrode voltage and the inverse of the welding current amplitude, and the time duration of each pulse corresponding to the other of the electrode voltage amplitude and the inverse of the welding current amplitude; and

averaging the amplitude of each such pulse over the pulse repetition time for producing a signal of amplitude corresponding to the contact resistance at the welding position.

2. A method for determining the instantaneous welding contact resistance of a resistance welder, comprising the steps:

generating first and second signals proportional, respectively, to the electrode voltage and welding current; producing a timing pulse in each of selected half cycles of the weld current;

measuring the instantaneous amplitude of said first and second signals each time a timing pulse occurs; producing at least one further pulse in each of said selected half cycles having a duration corresponding to the inverse of the measured amplitude of one of said first and second signals and an amplitude equal to the measured amplitude value in the other of said first and second signals; and

averaging the amplitude of said pulse over a preselected length of time to obtain a voltage representation of the welding contact resistance.

3. A method for determining the instantaneous welding contact resistance of resistance welder, comprising the steps:

sensing the electrode voltage and weld current at the weld position and generating first and second signals proportional, respectively, to the electrode voltage and welding current; detecting the point in each half cycle of the welding current at which weld current amplitude is maximized and producing a timing pulse in each half cycle at said point;

measuring the instantaneous amplitude of said first and second signals each time a timing pulse occurs and applying the amplitudes so measured to first and second memory means, respectively;

converting the inverse of the amplitude value in one of said first and second memory means, respectively;

converting the inverse of the amplitude value in one of said first and second memory means to a proportional time value;

producing a series of further pulses in each half cycle each having a duration equal to said time value and an amplitude equal to the amplitude value in the other of said first and second memory means; and

averaging the amplitude of said pulse over the length of a half cycle of welding current to obtain a DC voltage corresponding in magnitude to the welding contact resistance.

4. A circuit for determining the instantaneous welding contact resistance of a resistance welder, comprising the combination:

means for generating first and second signals proportional,

respectively, to the electrode voltage and welding current;

means for producing a timing pulse in each of selected half cycles of the weld current; means for measuring the instantaneous amplitude of said first and second signals each time a timing pulse occurs;

means for producing at least one further pulse in each of said selected half cycles having a duration corresponding to the inverse of the measured amplitude of one of said first and second signals and an amplitude equal to the measured amplitude value in the other of said first and second signals; and

means for averaging the amplitude of said pulse over a preselected length of time to obtain a voltage representation of the welding contact resistance.

5. A circuit for determining the instantaneous contact resistance of a resistance welder, comprising in combination;

means for sensing the electrode voltage and the welding current waveforms of the welder;

peak detecting means responsive to the occurrence of am plitude peaks of the welding current waveform for providing pulses concurrent therewith; first sampling means energized from said welding current sensing means and responsive to the pulse output from saidpeak detecting means for storing a signal proportional to the peak amplitude of the welding current;

second sampling means energized from'said electrode voltage sensing means and responsive to the pulse output from said peak detecting means for storing a signal proportional to the amplitude of the electrode voltage at the peak of the corresponding welding current half wave; and

quotient means supplied with said stored signals for produc ing a pulse train, the amplitude of the pulses of said train being proportional to the electrode voltage and the duration of the pulses of said train being inversely proportional to the peak amplitude of the welding current, said quotient means including means for averaging the amplitude of said pulses of said train, the resulting average value being proportional to the contact resistance at the welding position.

6. The device defined in claim in which said peak detecting means includes pulseproducing means and means responsive to failure of the welding current waveform to increase further in magnitude for causing said pulse producing means to produce a pulse when the weld current waveform peaks.

7. The device defined in claim 5 in which said peak detecting means includes a trigger circuit, means responsive to occurrence of the welding current peak for deenergizing said trigger circuit to cause a pulse to be produced thereby and a swamping circuit connected to the input of said trigger circuit for insuring energization thereof at the beginning of weld current flow at each half cycle of the weld interval.

8. The device defined in claim 5 in which said peak detecting means comprises rectifier means for rectifying the output of said weld current sensing means, a trigger circuit and capacitive means connecting the output of said rectifier to the input of said trigger circuit for energizing said trigger circuit from said rectifier means from the beginning of weld current flow until the peak amplitude thereof by conduction through said capacitor whereafter said capacitor ceases to energize the trigger circuit; a monostable multivibrator connected to the output of said trigger circuit and responsive to deenergization of said trigger circuit for producing an output pulse at the peak point of the welding current waveform.

9. The device defined in claim 5 in which said first sampling means comprises a gate circuit connected to the output of said peak detecting means for energization at the peak point of the welding current waveform, signal means energized from said welding current sensing means for providing a signal proportional in amplitude to the welding current and storage means, said gate circuit being connected between said signal means and said storage means for applying said signal to said storage means at the peak of the welding current waveform whereby said storage means stores a value proportional to the peak value of the welding current waveform each time the gate is energized.

10. The device defined in claim 5 in which said first sampling means comprises rectifier means energized by said means for sensing the welding current waveform, a storage capacitor and a gate circuit connected between said storage capacitor and the output of said rectifier, said gate being operatively connected to the output of said peak detecting means whereby the gate connects said rectifying means to said storage capacitor at the peak point of the welding current wavefonn to impress on said capacitor a charge proportional to the welding current peak amplitude of each half cycle of the welding current waveform; buffer means driven by said capacitor for providing a bufiered output corresponding to the peak amplitude of the welding current waveform.

11. The device defined in claim 5 in which said second sampling means comprises a gate circuit connected to the output of said ak detecting means for energization at the peak point 0 the welding current waveform, signal means energized from said electrode voltage sensing means for providing a signal proportional inamplitude to the electrode voltage and storage means, said gate circuit being connected between said signal means and said storage means for applying said signal to said storage means at the peak of the welding current waveform whereby said storage means stores a value proportional to the peak value of the electrode voltage waveform each time the gate is energized.

12. The device defined in claim 5 in which said second sampling means comprises rectifier means energized by said means for sensing the electrode voltage waveform, a storage capacitor and a gate circuit connected between said storage capacitor and the output of said rectifier means, said gate being operatively connected to the output of said peak detecting means whereby the gate connects said rectifier means to said storage capacitor at the peak point of the welding current waveform to impress on said capacitor a charge proportional to the electrode voltage amplitude at the peak of the welding current waveform; bufier means driven by said capacitor for providing a buffered output corresponding to the amplitude of the electrode voltage waveform. I

13. The device defined in claim 5 in which said quotient means includes timing means energizable at a rate proportional to the welding current peak magnitude;

an oscillator and means energizable by said oscillator and timing means for producing an alternating potential of frequency detennined by said oscillator the duration of one of the alternate potential levels being determined by said timer means; and

means responsive to the output of said second sampling means and to said alternating potential for producing said pulse train, in which pulse duration equals said duration of said one potential level.

14. in a resistance welder which includes means for comparing a value proportional to the contact resistance at the welding position during welding to a resistance function which it is desired that the actual contact resistance follow and further means for adjusting the welding heat to minimize the difference between the measured and desired resistances, a circuit for determining the instantaneous contact resistance at the welding position of the resistance welder, 'said circuit comprising the combination;

means for sensing the amplitude of the electrode voltage and welding current at the peak of the welding current wavefonn; and

means generating a pulse train and further means driven by said sensing means for rendering the amplitude and duration of pulses in said pulse-train proportional, respectively to the electrode voltage and welding current amplitudes at the welding current peaks whereby the average value pulse train amplitude is proportional to the contact resistance in the welding position.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 573 421 Dated April 6, 1971 Invent0 Peter W. Vanderhelst It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 12, lines 37 and 38, cancel beginning with "converting the inverse" to and including "means,

respectively;".

Signed and sealed this 2nd day of November 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,J'R. ROBERT GOT'I'SCHALK Attesting Officer Acting Commissioner of Pet 

1. A method for finding the contact resistance of a resistance welder comprising the steps: sensing the amplitude of the electrode voltage and welding current; producing a train of pulses, the amplitude of each pulse corresponding to one of the amplitude of the electrode voltage and the inverse of the welding current amplitude, and the time duration of each pulse corresponding to the other of the electrode voltage amplitude and the inverse of the welding current amplitude; and averaging the amplitude of each such pulse over the pulse repetition time for producing a signal of amplitude corresponding to the contact resistance at the welding position.
 2. A method for determining the instantaneous welding contact resistance of a resistance welder, comprising the steps: generating first and second signals proportional, respectively, to the electrode voltage and welding current; producing a timing pulse in each of selected half cycles of the weld current; measuring the instantaneous amplitude of said first and second signals each time a timing pulse occurs; producing at least one further pulse in each of said selected half cycles having a duration corresponding to the inverse of the measured amplitude of one of said first and second signals and an amplitude equal to the measured amplitude value in the other of said first and second signals; and averaging the amplitude of said pulse over a preselected length of time to obtain a voltage representation of the welding contact resistance.
 3. A method for determining the instantaneous welding contact resistance of resistance welder, comprising the steps: sensing the electrode voltage and weld current at the weld position and generating first and second signals proportional, respectively, to the electrode voltage and welding current; detecting the point in each half cycle of the welding current at which weld current amplitude is maximized and producing a timing pulse in each half cycle at said point; measuring the instantaneous amplitude of said first and second signals each time a timing pulse occurs and applying the amplitudes so measured to first and second memory means, respectively; converting the inverse of the amplitude value in one of said first and second memory means, respectively; converting the inverse of the amplitude value in one of said first and second memory means to a proportional time value; producing a series of further pulses in each half cycle each having a duration equal to said time value and an amplitude equal to the amplitude value in the other of said first and second memory means; and averaging the amplitude of said pulse over the length of a half cycle of welding current to obtain a DC voltage corresponding in magnitude to the welding contact resistance.
 4. A circuit for determining the instantaneous welding contact resistance of a resistance welder, comprising the combination: means for generating first and second signals proportional, respectively, to the electrode voltage and welding current; means for producing a timing pulse in each of selected half cycles of the weld current; means for measuring the instantaneous amplitude of said first and second signals each time a timing pulse occurs; means for producing at least one further pulse in each of said selected half cycles having a duration corresponding to the inverse of the measured amplitude of one of said first and second signals and an amplitude equal to the measured amplitude value in the other of said first and second signals; and means for averaging the amplitude of said pulse over a preselected length of time to obtain a voltage representation of the welding contact resistance.
 5. A circuit for determining the instantaneous contact resistance of a resistance welder, comprising in combination; means for sensing the electrode voltage and the welding current waveforms of the welder; peak detecting means responsive to the occurrence of amplitude peaks of the welding current waveform for providing pulses concurrent therewith; first sampling means energized from said welding current sensing means and responsive to the pulse output from said peak detecting means for storing a signal proportional to the peak amplitude of the welding current; second sampling means energized from said electrode voltage sensing means and responsive to the pulse output from said peak detecting means for storing a signal proportional to the amplitude of the electrode voltage at the peak of the corresponding welding current half wave; and quotient means supplied with said stored signals for producing a pulse train, the amplitude of the pulses of said train being proportional to the electrode voltage and the duration of the pulses of said train being inversely proportional to the peak amplitude of the welding current, said quotient means including means for averaging the amplitude of said pulses of said train, the resulting average value being proportional to the contact resistance at the welding position.
 6. The device defined in claim 5 in which said peak detecting means includes pulse producing means and means responsive to failure of the welding current waveform to increase further in magnitude for causing said pulse producing means to produce a pulse when the weld current waveform peaks.
 7. The device defined in claim 5 in which said peak detecting means includes a trigger circuit, means responsive to occurrence of the welding current peak for deenergizing said trigger circuit to cause a pulse to be produced thereby and a swamping circuit connected to the input of said trigger circuit for insuring energization thereof at the beginning of weld current flow at each half cycle of the weld interval.
 8. The device defined in claim 5 in which said peak detecting means comprises rectifier means for rectifying the output of said weld current sensing means, a trigger circuit and capacitive means connecting the output of said rectifier to the input of said trigger circuit for energizing said trigger circuit from said rectifier means from the beginning of weld current flow until the peak amplitude thereof by conduction through said capacitor whereafter said capacitor ceases to energize the trigger circuit; a monostable multivibrator connected to the output of said trigger circuit and responsive to deenergization of said trigger circuit for producing an output pulse at the peak point of the welding current waveform.
 9. The device defined in claim 5 in which said first sampling means comprises a gate circuit connected to the output of said peak detecting means for energization at the peak point of the welding current waveform, signal means energized from said welding current sensing means for providing a signal proportional in amplitude to the welding current and storage means, said gate circuit being connected between said signal means and said storage means for applying said signal to said storage means at the peak of the welding current waveform whereby said storage means stores a value proportional to the peak value of the welding current waveform each time the gate is energized.
 10. The device defined in claim 5 in which said first sampling means comprises rectifier means energized by said means for sensing the welding current waveform, a storage capacitor and a gate circuit connected between said storage capacitor and the output of said rectifier, said gate being operatively connected to the output of said peak detecting means whereby the gate connects said recTifying means to said storage capacitor at the peak point of the welding current waveform to impress on said capacitor a charge proportional to the welding current peak amplitude of each half cycle of the welding current waveform; buffer means driven by said capacitor for providing a buffered output corresponding to the peak amplitude of the welding current waveform.
 11. The device defined in claim 5 in which said second sampling means comprises a gate circuit connected to the output of said peak detecting means for energization at the peak point of the welding current waveform, signal means energized from said electrode voltage sensing means for providing a signal proportional in amplitude to the electrode voltage and storage means, said gate circuit being connected between said signal means and said storage means for applying said signal to said storage means at the peak of the welding current waveform whereby said storage means stores a value proportional to the peak value of the electrode voltage waveform each time the gate is energized.
 12. The device defined in claim 5 in which said second sampling means comprises rectifier means energized by said means for sensing the electrode voltage waveform, a storage capacitor and a gate circuit connected between said storage capacitor and the output of said rectifier means, said gate being operatively connected to the output of said peak detecting means whereby the gate connects said rectifier means to said storage capacitor at the peak point of the welding current waveform to impress on said capacitor a charge proportional to the electrode voltage amplitude at the peak of the welding current waveform; buffer means driven by said capacitor for providing a buffered output corresponding to the amplitude of the electrode voltage waveform.
 13. The device defined in claim 5 in which said quotient means includes timing means energizable at a rate proportional to the welding current peak magnitude; an oscillator and means energizable by said oscillator and timing means for producing an alternating potential of frequency determined by said oscillator the duration of one of the alternate potential levels being determined by said timer means; and means responsive to the output of said second sampling means and to said alternating potential for producing said pulse train, in which pulse duration equals said duration of said one potential level.
 14. In a resistance welder which includes means for comparing a value proportional to the contact resistance at the welding position during welding to a resistance function which it is desired that the actual contact resistance follow and further means for adjusting the welding heat to minimize the difference between the measured and desired resistances, a circuit for determining the instantaneous contact resistance at the welding position of the resistance welder, said circuit comprising the combination; means for sensing the amplitude of the electrode voltage and welding current at the peak of the welding current waveform; and means generating a pulse train and further means driven by said sensing means for rendering the amplitude and duration of pulses in said pulse train proportional, respectively to the electrode voltage and welding current amplitudes at the welding current peaks whereby the average value pulse train amplitude is proportional to the contact resistance in the welding position. 